The invention relates to a method of printing on a rotary printing press having a plurality of printing cylinders that are adapted to be adjusted against a web of a printing medium.
In conventional printing presses of this type, the plurality of printing cylinders serve for printing in several colours. For example, flexographic printing presses are known, in which a plurality of ink units which each comprise a printing cylinder are arranged around the periphery of a common back pressure cylinder. The web passes around the back pressure cylinder and is printed successively in the individual ink units with the colour components of the image to be printed. Since the printed image is repeated after each rotation of the printing cylinder, the printing length, i.e. the length of the image to be printed in feed direction of the web, is limited by the peripheral length of the printing cylinder. Thus, for printing images in a large format, a printing press is required in which the printing cylinders have a large diameter so as to admit a large printing length and/or have a large axial length, so as to permit a correspondingly large printing width for the image to be printed in transverse format. In both cases, an expensive printing press is required, which has such a construction that large printing lengths and/or printing widths are permitted.
It is an object of the invention to provide a method permitting to print images with a large format on a compact and inexpensive printing press.
According to the invention, this object is achieved by the features that the printed image is divided into a plurality of elements which are printed with different printing cylinders, and that at least one of these printing cylinders is periodically lifted off from the web during the printing operation, each time for a duration corresponding to at least one turn of the printing cylinder.
As a consequence, the peripheral length of the printing cylinder does not have to be equal to the total length of the printed image, but is only required to have the length of the element to be printed with this printing cylinder. When one element of the image has been printed during one turn of the printing cylinder, this printing cylinder is lifted off from the web for at least the duration of one turn, so that the corresponding site on the web is left empty. Then, another element of the image is printed on this empty site by means of another printing cylinder. In this operation, the longitudinal register is selected such that the elements printed one after the other with different printing cylinders are in registry and are combined to form the desired printed image. The maximum format of the printed image that can be printed with the printing press in this way is accordingly limited in one direction by the printing width of the machine, but can, in the other direction, amount to a multiple of the peripheral length of the printing cylinder, depending on the number of printing cylinders being used.
Thus, the method permits a very variable use of the printing press. When the desired printing length is not larger than the peripheral length of the largest printing cylinder, each printing cylinder can be used for another colour component, so that multi-colour printing with a number of colours corresponding to the number of printing cylinders is enabled. As an alternative, however, larger image formats can be printed by using at least two printing cylinders for different elements of the same image. If these elements are to be printed in the same colour or have a common colour component, however, the number of available printing colours is limited, accordingly. This, however, is well acceptable for many practical applications.
For instance, one example of industrial applicability is the printing of large-format tablecloths, e.g. disposable paper tablecloths. On a printing press having six ink units for example, it is then possible to print with three colours and with a printing length corresponding to twice the peripheral length of the printing cylinders.
Another practically important example of applicability is the printing of packaging material, in which the printed images for the individual panels, each of which will later form a single package, have common image elements for one part and different image elements for the other part. The different image elements may for example comprise different type-identifiers, a printed packaging date and the like. If, in a conventional printing method, the peripheral length of the printing cylinders corresponds to three times the length of a panel, for example, then not more than three panels with different image elements can be printed in one and the same run. If a printing press with four ink units is used, but only two colours are needed for the printed image, e.g. one colour for the common single-colour image elements and a second colour for a variable imprint, then two ink units of the printing press remain unused. Now, in the method according to the invention, it is possible to utilise also the two idle ink units for printing imprints. Each printing cylinder of these two ink units can then again produce three different imprints, so that a totality of nine different imprints can be printed in a single run in a very efficient way.
The invention further provides a printing press that is suitable for carrying out the method described above. As is known per-se, the printing press has a shift mechanism for shifting the printing cylinders between a position in which they are engaged against the web and a position in which they are lifted off from the web. According to the invention, this printing press has a control unit adapted to shift at least one of the printing cylinders periodically against and off the web during the printing operation.
Preferably, the shift mechanism is designed such that the length of time needed for shifting the printing cylinder between the engaged the non-engaged position is substantially smaller than the rotation period of the printing cylinder. In conventional flexographic printing presses, such shift mechanisms serve for shifting an anilox roller, that is used for inking the printing cylinder, relatively far away from the printing cylinder and for shifting the printing cylinder relatively far away from the back pressure cylinder or central cylinder, thereby to provide sufficient space for handling the cylinders when the cylinders have to be exchanged, for example. In the printing press according to the invention, however, when the printing cylinder is periodically shifted between the engaged and non-engaged position during the printing operation, this printing cylinder only needs to be lifted off from the web to such an extent that ink is no longer transferred onto the web. To this end, a distance of a fraction of a millimeter is sufficient. Even in case of high speed printing presses, the time needed for shifting may therefore be negligible in comparison to the rotation period of the printing cylinder, so that the image elements printed by different printing cylinders may be butted precisely and seamlessly and essentially without overlap.
The shifting mechanism may have a known construction, for example, in the form of ball bearing-type spindle/nut units with servo motors, by which the displacements may be controlled very precisely, so that the original position may be restored with high accuracy. These drive mechanisms act upon brackets in which the printing cylinders are journalled and which are guided for example on linear guides with ball bearings.
For achieving, on the one hand, large displacements in case that a cylinder has to be changed, and, on the other hand, small displacements in the periodic shifting operations, a two-stage drive system may also be used. The long-stroke displacements are then achieved by a first stage, whereas the second stage is provided for the shifting movements having only a very short stroke. In this case, the short-stroke movements may also be limited by stops. The drive mechanisms may as well be formed by hydraulic or pneumatic actuators or piezoelectric actuators, for example.
In general, it will be advantageous if at least two printing cylinders can be shifted periodically. In special applications, for example, when the printed image consists only of two elements, i.e. a relatively short imprint and a background in a uniform colour or with a periodic pattern, it is sufficient that only the printing cylinder for the imprint can be shifted.